browser.webNavigation.onReferenceFragmentUpdated.addListener(
  listener,                   // function
  filter                      // optional object
)
browser.webNavigation.onReferenceFragmentUpdated.removeListener(listener)
browser.webNavigation.onReferenceFragmentUpdated.hasListener(listener)

Events have three functions:

addListener(callback)

Adds a listener to this event.

removeListener(listener)

Stop listening to this event. The listener argument is the listener to remove.

hasListener(listener)

Check whether listener is registered for this event. Returns true if it is listening, false otherwise.

callback

Function that will be called when this event occurs. The function will be passed the following arguments:

details

object. Details about the navigation event. See the details section for more information.

filter Optional

object. An object containing a single property url, which is an Array of events.UrlFilter objects. If you include this parameter, then the event will fire only for transitions to URLs which match at least one UrlFilter in the array. If you omit this parameter, the event will fire for all transitions.

tabId

integer. The ID of the tab in which the navigation is about to occur.

url

string. The URL to which the given frame will navigate.

processId

integer. The ID of the process in which this tab is being rendered.

frameId

integer. Frame in which the navigation will occur. 0 indicates that navigation happens in the tab's top-level browsing context, not in a nested <iframe>. A positive value indicates that navigation happens in a nested iframe. Frame IDs are unique for a given tab and process.

timeStamp

number. The time that the fragment identifier for the page was changed, in milliseconds since the epoch.

transitionType

transitionType. The reason for the navigation: for example, "link" if the user clicked a link.

transitionQualifiers

Array of transitionQualifier. Extra information about the navigation: for example, whether there was a server or client redirect.

Logs the target URLs and extra transition information for onReferenceFragmentUpdated, if the target URL's hostname contains "example.com" or starts with "developer".

const filter = {
  url:
  [
    {hostContains: "example.com"},
    {hostPrefix: "developer"}
  ]
}

function logOnReferenceFragmentUpdated(details) {
  console.log(`onReferenceFragmentUpdated: ${details.url}`);
  console.log(`Transition type: ${details.transitionType}`);
  console.log(`Transition qualifiers: ${details.transitionQualifiers}`);
}

browser.webNavigation.onReferenceFragmentUpdated.addListener(logOnReferenceFragmentUpdated, filter);